The lignocellulosic biomass jute fiber (JF) was chemically modified with polyphenolic tannin in aqueous medium by epoxy activation under mild conditions and applied as a potential adsorbent for the removal of Congo Red, a model azo dye, from aqueous solution. The virgin and tannin-modified JF samples were characterized by solid-state nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. Within the studied range of dye concentrations, the adsorption equilibrium was found to follow the Langmuir isotherm model well, with R-2 > 0.99. The rate of adsorption of the dye onto treated JF was very high, and equilibrium was attained within 15-30 min of contact. The efficiency of modified JF for the spontaneous and exothermic adsorption of azo dye is attributed to the copious availability of hydroxyl and other polar functional groups on the fiber surface. The present adsorption studies of azo dye from aqueous solution revealed the potential of modified JF to be utilized as an alternative, inexpensive, and environmentally benign adsorbent for water purification.